The present invention relates to a fluid flow rate measuring apparatus using a hot-wire of thermosensitive resistive material and, particularly, to a hot-wire flow rate measuring apparatus for measuring the flow rate of intake air in an automobile engine.
Automobile engines are required to control accurately the air-fuel ratio and ignition timing so as to maintain a lower toxicity of exhuast emission and fuel consumption rate, and for this purpose microcomputerized engine control systems have already been introduced. In such systems, the accuracy of measuring the intake air mass, i.e., intake air flow rate, determines the engine performance, and therefore the accurate flow rate measurement is particularly requested.
For measuring the fluid flow rate, there is known a hot-wire flow rate measuring technique applicable to the automobile engine air flow sensor, in which a heated thermo-sensitive resistive element is exposed to the fluid in its flow path and the flow rate is detected electrically based on the heat transfer characteristics pertaining to the fluid flow rate and the heating value of the resistive element, as disclosed for example in U.S. Pat. Nos. 3,747,577 and 4,297,881.
FIG. 1 shows the fundamental circuit arrangement of such a conventional flow rate measuring apparatus. A d.c. voltage source 1 supplies a current through the collector-emitter junction of a transistor 2 to a serial connection of a thermo-sensitive resistive element 3 and a resistor 4. Another serial connection of a resistor 5, a thermal compensation thermo-sensitive resistive element 6 and a resistor 7 is connected between the emitter of the transistor 2 and the negative terminal of the voltage source 1. The node of the sensing element 3 and resistor 4 and the node of the compensation sensing element 6 and resistor 7 provide a non-inverted input and inverted input for an amplifier 8, which has its output connected to the base of the transistor 2. The sensing element 3 is for the flow rate measurement and is disposed in the stream of fluid, while the sensing element 6 is placed in the flow path so as to detect the fluid temperature. The sensing element 3, resistors 4 and 5, compensation sensing element 6, and resistor 7 have respective resistances R.sub.3, R.sub.4, R.sub.5, R.sub.6, and R.sub.7. Assuming the sensing elements 3 and 6 to have an equal temperature coefficient .alpha., their resistances are expressed as follows. EQU R.sub.3 =R.sub.30 (1+.alpha.T.sub.3) (1) EQU R.sub.6 =R.sub.60 (1+.alpha.T.sub.6) (2)
where T.sub.3 and T.sub.6 are temperatures of elements 3 and 6, R.sub.30 and R.sub.60 are resistances of elements 3 and 6 at the reference temperature.
The bridge circuit made up the components 3-7 has the equilibrium condition expressed as, EQU R.sub.7 .multidot.R.sub.3 =R.sub.4 .multidot.(R.sub.5 +R.sub.6) (3)
The above equations (1), (2) and (3) are combined to give, ##EQU1## where .DELTA.T=T3-T6
It is known that the heat produced by an electric current flowing in a heated body, i.e., the sensing element 3, placed in a flowing fluid is carried away by the fluid as expressed by the following equation. EQU Q=I.sup.2 R.sub.3 =(C.sub.1 +C.sub.2 .sqroot.U).DELTA.T (5)
where C.sub.1 and C.sub.2 are constants, Q is heating value, I is current in resistive element 3, and U is the mass air flow rate per unit time.
Namely, when the differential temperature .DELTA.T between the heating element and the fluid is constant, the heating value is proportional to the root of the air flow rate. By making the factor of T6 equal to zero in equation (4) i.e., R.sub.4 .multidot.R.sub.60 /R.sub.7 .multidot.R.sub.30 =1, the differential temperature .DELTA.T becomes a constant determined from the circuit condition, and then it is possible to evaluate the flow rate by measuring the heating value Q. Thus, equation (4) is reduced to as follows. ##EQU2##
A problem of this method is that the temperature compensating thermo-sensitive resistive element 6 is heated by the current flowing in it, causing an error in the differential temperature .DELTA.T. The heating values produced by the resistors 3 and 6 are dependent on their terminal voltage. As can be seen from the circuit configuration of FIG. 1, the voltage applied across the sensing element 3 is substantially equal to the voltage applied across the serial connection of the resistor 5 and compensating element 6. In order to eliminate the effect of heating of the compensating element 6, it must have applied thereto a voltage creating a negligibly small amount of heat in it, while a voltage adequate to heat the sensing element 6 is applied to it. The ratio of resistances of the elements 5 and 6 is fixed by the equation (6), and therefore in order for the compensating element 6 to have a sufficiently small amount of heat generation, its resistance must be large enough as compared with the resistance of the sensing element 3. Manufacturing of thermo-sensitive resistive elements having greatly different resistances using the same material (e.g., platinum wire) is generally uneconomical and also likely to invite disparities of properties (e.g., temperature coefficient .alpha.) during the manufacturing process.
On this account, hot-wire flow rate measurement is accompanied by many fluctuation factors of the heat transfer characteristics that relate the sensing element with the fluid. Accordingly, minimizing the disparity of the flow rate to output voltage characteristics among flow rate measuring devices is a technical theme in the mass production of automobile parts in the automotive industry.